Radio frequency (RF) power amplifiers are used in wireless communication networks for the transmission of signals. Solid state RF power amplifiers can be modeled by amplitude-modulation/amplitude-modulation (AM/AM) characteristics, because amplitude-modulation/phase-modulation (AM/PM) characteristics are generally negligible. As the amplitude of an input signal of an amplifier is increased, the output signal will begin to saturate at some level in the RF amplifier. The amplitude where the output signal moves from a linear region to a saturation region is often referred to as the one decibel (1 dB) compression point of the amplifier.
Different models for RF power amplifiers have been developed, and an important feature of these models is the manner in which the output signal moves from the linear region to the saturation region. To maintain linear operation, power amplifiers are usually backed off by some number of dBs from the one decibel (1 dB) compression point. The required back-off depends on the crest factor (CF) of the input signal. For orthogonal frequency division multiplexing (OFDM) modulation, back-offs higher than 5 dB are generally used. Typical back-off numbers are in the region of 9-12 dB for 64 carrier OFDM (64 OFDM) signals.
As known by those having ordinary skill in the art, the crest factor (CF) of a signal can be defined as the peak to average amplitude ratio. A peak to average power ratio (PAPR) can be computed from the CF. High CF multi-carrier modulation, such as OFDM, can pose problems for RF power amplifiers because the amplifiers require high linearity to reproduce the high peak powers of a signal without distortion.
The high CF of OFDM results from individual carrier components being added together at different phases in an Inverse Fast Fourier Transform (IFFT). The CF is relatively independent of the modulation method of the individual carriers when there are many sub carriers. The CF is also relatively independent of the number of sub carriers for practical 32-256 sub carrier OFDM modulations.
If the CF of a signal can be reduced prior to inputting the signal to an RF power amplifier, the back-off can be reduced and mean output power can be increased. Numerous CF reduction methods have thus been developed. Generally, these include selective mapping, where multiple information equivalent signals are created and the signal with the lowest CF is selected for transmission. Partial transmit signal techniques use a similar approach, where multiple partial signals are generated and the most beneficial linear combination is transmitted. In addition, hard and soft clipping methods are used to limit CF by removing or reducing peaks.
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